Name

    QCOM_texture_lod_bias

Name Strings

    GL_QCOM_texture_lod_bias

Contact

    Rob VanReenen (rvanreen 'at' qti.qualcomm.com)

Contributors

    Caiqin Li
    Rob VanReenen
    Jeff Leger
    Tate Hornbeck

Status

    Draft.

Version

    Last Modified Date: September 6, 2023
    Revision: 01

Number

    OpenGL ES Extension #342

Dependencies

    OpenGL ES 2.0 or higher is required.

    This extension is written against the OpenGL ES
    Version 3.2 (May 5, 2022).

Overview

    This extension reintroduces the texture LOD bias operation from
    EXT_texture_lod_bias which is written based on OpenGL ES 1.1.
    It provides a mean to bias the lambda computation by a
    constant (signed) value. This bias can provide a way to blur or
    pseudo-sharpen OpenGL ES's standard texture filtering.

    This blurring or pseudo-sharpening may be useful for special
    effects (such as depth-of-field effects) or image processing
    techniques (where the mipmap levels act as pre-downsampled image
    versions). On some implementations, increasing the texture LOD
    bias may improve texture filtering performance (at the cost of
    texture blurriness).

IP Status

    No known IP claims.

New Procedures and Functions

    None

New Types

    None

New Tokens

    Accepted by the <pname> parameter of TexParameterf,
    SamplerParameterf, GetTexParameterfv and GetSamplerParameterfv:

        TEXTURE_LOD_BIAS_QCOM                  0x8C96

Additions to Chapter 3 of the 3.0 Specification (Rasterization)

 -- Section 8.10 "Texture Parameters"

    Add following to Table 8.19: Texture parameters and their values.

    Name                    Type            Legal Values
    TEXTURE_LOD_BIAS_QCOM   float           any value

 -- Section 8.14 "Texture Minification"

    Change the formula (8.7) under "Scale Factor and Level of Detail"
    to read:

    "lambda'(x,y) = lambda_base(x,y) + clamp(clamp(bias_shader) + clamp(lodBias))"

    Add the third paragraph under "Scale Factor and Level of Detail":

    "where lodBias is the texture unit's (signed) texture LOD bias
    parameter clamped between the positive and negative values of the
    implementation defined constant MAX_TEXTURE_LOD_BIAS_EXT."

Errors

    None

New Implementation Dependent State

    Add to Table 21.10: Textures (state per texture object)

    Get Value              Type  Get Command        Initial Value   Description       Sec.
    ---------------------  ----  -----------        --------------  ----------------  -----
    TEXTURE_LOD_BIAS_QCOM  R     GetTexParameterfv  0.0             Texture LOD bias  8.14

    Add to Table 21.12: Textures (state per sampler object)

    Get Value              Type  Get Command            Initial Value   Description       Sec.
    ---------------------  ----  -----------            --------------  ----------------  -----
    TEXTURE_LOD_BIAS_QCOM  R     GetSamplerParameterfv  0.0             Texture LOD bias  8.14

Issues

    (1) Does this extension allow the same texture object to be used
        by two different texture units for different LOD biases?

        RESOLUTION: It works if you use different sampler objects bound
        to the same texture object. If you set the default sampler
        object via TexParameterf, you can only set one LOD bias as one
        texture has only one default sampler.

    (2) How does QCOM_texture_lod_bias differ from SGIX_texture_lod bias?

        QCOM_texture_lod_bias adds a bias to lambda. The
        SGIX_texture_lod_bias extension changes the computation of rho
        (the log2 of which is lambda). The SGIX extension provides
        separate biases in each texture dimension. The QCOM extension
        does not provide any "directionality" in the LOD control.

    (3) Does the texture LOD bias occur before or after the TEXTURE_MAX_LOD
        and TEXTURE_MIN_LOD clamping?

        RESOLUTION: BEFORE. This allows the texture LOD bias to still
        be clamped within the max/min LOD range.

Revision History

    Rev.    Date       Author      Changes
    ----  --------     --------    --------------------------
    #01    9/6/2023   Caiqin Li   Initial draft.